The present invention relates to integrated circuit transceivers for use with transmission lines, in particular, transceivers complying with CCITT recommendation V.35 which are configurable in either a DTE or DCE mode.
Computer terminal equipment communicates with remote access devices via a differential, twisted-pair line. A standard for one such configuration is CCITT V.35. This standard sets forth a differential pair for both transmission of data and reception of data, along with a transmission clock, a receive clock, and a transmit clock echo. The device originating the transmission and the original clock is referred to as the Data Communication Equipment (DCE) and the end of the transmission line receiving the transmissions and the clock is referred to as the Data Terminal Equipment (DTE). Transceiver integrated circuits have been developed to provide the transmitters and receivers needed for the interface at both ends of the transmission line to convert from CMOS/TTL input levels into a differential pair for transmission.
In the loopback diagnostic mode, it is desirable for either end to be able to send out and receive back signals to test both its own output and the transmission line. This can be accomplished with multiple discrete logic devices to intercept the signal at the other end and reroute it back over the transmission line on the appropriate path. Different discrete logic is required for both the DTE and DCE ends because of the different paths that are looped back. For instance, the DCE always originates the transmission clock, and the DTE is the one that normally provides the clock echo back in normal operation.